The measurement of resistances or resistors, particularly those formed by strain gauges, which for measurement purposes are arranged in the form of a measuring bridge, particularly a Wheatstone bridge, reveal a temperature variation, i.e. the resistance value changes with the temperature. A pressure cell or pressure sensor with such resistance strain gauges e.g. has as the pressure-sensitive element a silicon-based, thin diaphragm. If the latter is etched from the silicon body and locally provided with foreign atoms diffused in to or implanted in the silicon crystal, zones are formed with a modified conductivity appearing electrically as conductors or local resistors. As soon as a pressure acts through a pressure duct on the measuring cell, there is a deformation of the thin silicon diaphragm.
As a result of the internal forces which occur, the molecular structure of the crystal is reversibly modified. Particularly in the diffused in resistance regions pronounced potential shifts occur in the crystal and lead to a measurable change of the electrical values. Through the wiring of the local resistance regions as a measuring bridge, when a voltage is applied pressure-dependent, electronic signals are obtained but, as stated, the resistance regions can have a temperature variation. Thus, the prior art method and device are consequently e.g. used in determining resistance values of such strain gauge arrangements and therefore for measuring strains, which are caused by forces, pressures, torques and weights and therefore for determining the latter quantities.
The aforementioned temperature variation of such sensors must be compensated in applications with a restricted temperature range (−10 . . . +40° C.), such as e.g. domestic or sales scales by adopting additional measures. For most industrial and also automotive applications the temperature range is higher (−40 . . . 125° C.) and the strain gauges must be carefully adapted to the field of use and are often provided with additional costs with a temperature measurement. In all cases production must be stable and reproducibly run in, so that the applications achieve the desired temperature variation without additional balancing measures. A subsequent balancing of the moulded measuring bridges is frequently no longer possible or gives rise to additional costs of a significant nature.
In the case of measuring bridges a constant voltage is applied across the bridge and a compensating resistor and the differential voltage between the bridge arms is measured.
The suppression of a thermal output signal without load is governed by the ratio formation of the resistors, but only if no differential voltage is applied at the bridge taps and consequently the balancing condition R1/R3=R2/R4 applies for the bridge resistors.
For example, the resistance value of all four resistors of a measuring bridge can change due to the thermal expansion of the bearing or contact surface. As, however, the thermal expansion uniformly applies to all four bridge resistors, the ratio of the resistors remains unchanged and consequently the balancing condition is fulfilled. With such bridges balancing resistors are introduced for the bridge zero balancing and are frequently mechanically balanced.
In the case of an unbalanced bridge with an expanded strain gauge further temperature dependences occur. With most resistors the gauge factor, i.e. the relationship between the expansion and the resulting resistance change, is temperature-dependent. With force measurements the modulus of elasticity, i.e. the relationship between the expansion of the bearing surface of the strain gauge and the measured force, is also temperature-dependent.
The temperature variation of the gauge factor or modulus of elasticity is not completely suppressed by the ratio formation of the resistance values in the case of a loaded Wheatstone bridge. A temperature-dependent and unbalance-dependent error remains in the measured bridge differential voltage, the so-called span error.
A method frequently used nowadays for minimizing the span error is the introduction of one or two series resistors into the voltage supply of the bridge. These series resistors have a temperature dependence which is opposed to that of the span error and e.g. with rising temperature as a result of the resistance increase reduces the effective bridge voltage and consequently keeps constant the bridge differential voltage. Very good results can be obtained with this method over a limited temperature range (e.g. −10° C. . . . +40° C.). However, for this purpose these compensating resistors must be adapted as precisely as possible to the circumstances of the strain gauges and load bodies.
In addition, due to unavoidable tolerances of the strain gauge resistors there is a bridge output signal without loading unless further balancing measures are taken, i.e. the offset error. To ensure that the span compensation does not lead to additional temperature-dependent offset errors, the bridge voltage without load must be balanced to very close to zero. If this was not the case, the bridge voltage reduction through the span resistance would reduce the offset voltage to the same extent. An offset voltage corrected to zero solely mathematically (e.g. in the connected microprocessor) would consequently fluctuate over the temperature.
In order to correct such errors the non-unbalanced, unloaded bridge voltage is balanced to zero volt and a matching compensating resistor is applied for compensating the bridge output signal. The electronics which then evaluate the bridge signal are not included in said balancing measures. A disadvantage of this procedure are the complicated, high demands made regarding the reproducibility and constancy of manufacture. In addition, the necessary balancing measures must be known prior to the manufacture of the measuring cells. Otherwise there would be a significant rise in the balancing costs following manufacture or a subsequent balancing is no longer possible.
It is also known for the subsequent balancing of the temperature variation through downstream electronics to measure both the differential voltage of the measuring bridge and the electrical voltage across the measuring bridge or a compensating resistor. Modern analog/digital converters (such as Cirrus CS5532, Melexis MLX90308) already have a further measuring channel for such a measurement.
A disadvantage of this method is the high input voltage range of the A/D converter between the potential of the compensation measurement and the bridge voltage potential. A high quality A/D converter with at least two measuring channels is required.
Thus, in this method there is an additional channel on the analog/digital converter and usually also an additional line to the measuring cell. This mathematical compensation method is rarely used due to the additional line and the increased demands on the A/D converter.
EP 1 251 357 A1 discloses the determination of resistances by measuring the discharge times of a capacitor across the same. Thus, in the case of temperature-dependent resistances or resistors temperature measurements can be carried out, if the temperature resistance characteristic is known, but there is no compensation of the temperature variation.
Based more particularly on the initially indicated prior art, the problem of the invention is to provide a method and a device with which account can be taken of and compensation can take place of a span error in measuring bridges, such as Wheatstone bridges, more particularly with strain gauges. A span error is not only caused by the resistors, but also by thermal expansion and the temperature variation of the modulus of elasticity of the bearing surface.